Spring Drive accuracy varies with temperature; they go a bit faster when warm than when cold. For instance, in the winter, if you spend time outside (or if your home is cooler), they slow down a bit. And the mechanism is somewhat warmer when on your wrist than in your sock drawer.

My SD Chrono is about 4 seconds a month fast when worn half the time, and about +7 seconds a month when worn all the time.

My GS SD GMT is about 2 seconds a month slow when worn half the time, and spot on when worn all the time.